Multi-stage pumps are widely used, particularly in combination with mass spectrometers because of the possibility of reducing the overall costs, size and complexity of the installation without endangering the high vacuum pump performance. It is conventional for the best vacuum and suction capability to be present at an inlet, while subsequent inlets provide a vacuum lying between the best vacuum and the external pressure. Two particularly important designs are known:                a multi-port split-flow pump, as disclosed in U.S. Pat. No. 6,464,451 and EP 0 603 694, with each inlet having an individual vacuum seal against atmospheric pressure.        a multi-port split-flow pump, as disclosed in U.S. Pat. No. 6,464,451 and EP 0 603 694, with each inlet having an individual vacuum seal against atmospheric pressure.        cartridge split-flow pumps, as disclosed in EP 1 422 423 and EP 1 090 231, with the pump being arranged within a suitable structure and being inserted into a housing with the latter. Typically, only the outlet in the vicinity of the pump outlet is provided with a seal against atmospheric pressure, while other outlets are only sealed against one another.        
Both approaches cause difficulties at an inlet of the pump or at two vacuum inlets as soon as an ultra high vacuum (UHV) is intended to be attained. Since UHV generally requires conflat (or other metallic) seals, it can be difficult to implement this in a pump with a number of inlets, in particular if two vacuum inlets should have a UHV. The metallic seals require utmost accuracy in the arrangement of the seal surfaces. Should a number of inlets with metallic seals be provided, these have to be matched to one another in a precision fitting manner. With a pump arranged in a cartridge, the baking for achieving the UHV conditions has to be carried out at significantly lower temperatures to avoid damage to the bearings (of the pump rotor). The invention is intended to overcome the described problems.